Polycyclic aromatic hydrocarbons (PAHs) are among the top 10 contaminants of concern at Superfund sites and are the major contaminants at an estimated 45,000 sites outside the jurisdiction of the Superfund program. The U.S. EPA regulates 16 PAHs, 7 of which are considered to be human carcinogens. PAHs are inherently biodegradable, but a significant fraction of each of the EPA-regulated PAHs is usually not removed during bioremediation of PAH-contaminated systems. Of particular concern is that the 4-, 5- and 6- ring compounds, which include all of the carcinogenic PAHs, are removed less extensively than the lower-molecular-weight compounds. Limitations in PAH biodegradation in field-contaminated soils are typically attributed to limitations in the availability of the PAHs to microorganisms, but this is rarely documented on a site-specific basis and has proven to be untrue of the 5- and 6-ring PAHs in a number of cases. Our ability to elucidate the factors that influence PAH degradation in any field-contaminated system is constrained by the overwhelming complexity of the system and by our relatively limited knowledge of microbial PAH metabolism and the diversity of PAH-degrading microorganisms. In some cases, interventions intended to improve PAH biodegradation can lead to the formation of toxic byproducts of PAH transformation by microorganisms. We propose to investigate the relationships among PAH bioavailability, biodegradation, the formation of toxic biotransformation products, and net risk reduction using two experimental platforms with field-contaminated soil. Soil column systems will be used to evaluate in situ approaches to bioremediation, and slurry-phase bioreactors will be used to study above-ground approaches to bioremediation. We hypothesize that different microbial communities will be selected in these systems, leading to differences in the ability to remove the most bioavailable fractions of the PAHs and in the propensity to form toxic byproducts. We also hypothesize that methods proposed to enhance PAH bioavailability for improved biodegradation are likely to form genotoxic products. Knowledge of the microbial ecology of PAH biodegradation will be expanded by using an emerging cultivation-independent molecular tool (stable-isotope probing) to identify microorganisms responsible for degrading a range of PAHs in relevant, complex systems. This knowledge is important in eventually being able to assess the potential for an indigenous microbial community to degrade specific PAHs in contaminated environments. Because dermal exposure is a major route of exposure to PAHs in contaminated soil, we will also collaborate with Project 4 to investigate the effects of contaminated soil on dermal exposure endpoints. Preliminary experiments will quantify the bioavailable fractions of PAHs relevant to potential uptake by human skin, and the effects of biological treatment of the soil on dermal exposure will be evaluated.